7.3 Powerstroke 4x4 X 18.5mpg down to 16 now?

I have a 2000 Excursion with 7.3 diesel, It was consistent for 3 years with 19mpg (US) but now it is hard to coax more than 16mpg, all at steady 65mph.
I did notice that if run for close to an hour it starts to get close to my 19mpg. It is almost like it is lugging during first hour of warm up.
Thermostat is ok, No OBDII fault codes, hubs are unlocked. It seams tempurature related, I'm guessing. Can the turbo waste gate be pulling down my milage? If anyone has any experience with this engine I would appreciate any pointers.

Ok i have given this more thought and ive come up with a coulpe of items that might have happened.

the fan clutch may not be completely disengauging. after the truck is at opterating temp turn it off and see if there is any resistance to turning it. if so replacing it will help. other simptom of a bad fan clutch would be that howl of rushing wind in the higher rpm band.

if the truck idles ruff you might consider having the fuel distibutor sent out for calibration.

tires and pressure make a big differecne, my tires are rated at 90 psi i run them at 70.

good luck

I have notice that my 6.0 does tend to lag more when it's cold, but this could be cold start protection by the ECM. I guess so you don't grenade anything.

I doubt that a wastegate is going to cost you fuel mileage, if it were that bad you'd have a performance concern first. Have you been keeping up on maintenace? A fuel filter or air filter can choke those 7.3s. Also, a bad u-joint or map sensor that fails but is in range, and not setting a code could be at fault.